Page 79 - Biology_F5

Biology for Advanced Level Secondary Schools

Pyruvic acid is converted into ethanal and carbon dioxide by breakage of its
double bond.

(e) Isomerases: Catalyse rearrangement within a molecule, converting one isomer to
another. For example, glucose-1-phosphate is converted into glucose- 6-phosphate
in the presence of the phospho-hexo-isomerase enzyme. The position of the
FOR ONLINE READING ONLY
phosphate group in the glucose-1-phosphate molecule is changed to form the
isomer glucose-6-phosphate.

CH OH CH OPO 2-
2 2 3
H O H Phospho-hexo-isomerase H O H
H H
OH H OH H
HO O OPO PO 2- 2- HO OH
3 3
H OH H OH
Glucose - 1 - phosphate Glucose - 6 - phosphate
Figure 1.51: Isomerisation of Glucose 1-phosphate to Glucose-6-phosphate

(f) Ligases: Catalyse the joining of two molecules by forming a new chemical bond,
and it requires energy from the breaking down of ATP. An example of such enzymes
is amino acyl-tRNA systhetase which catalyses the formation of amino acid-tRNA
complex during protein synthesis.

1.4.5 Adenosine Triphosphate (ATP)
Exercise 1.14 ATP is a nucleoside triphosphate used in

cells, often called the “molecular unit of
1. Describe the factors affecting currency” of intracellular energy transfer.
enzyme activity. It belongs to a category of high energy

2. Explain how the lock-and-key compounds that release energy when
hypothesis illustrates specificity of the bond between the second and third
enzymes. phosphate is broken. The presence of

3. Classify enzymes on the basis of these high energy bonds makes it possible
reactions they catalyse. for ATP to store and release energy for
cellular reactions.
4. Explain why substrate concentration
has no effect on non-competitive
inhibition. Chemical composition
ATP consists of adenosine (composed of
5. Explain why at temperatures of an adenine ring and a five carbon sugar,
above 40 °C, mammalian enzymes ribose sugar) and three phosphate groups
do not function efficiently.
(triphosphate) (Figure 1.52). ATP is
highly soluble in water and is quite stable
in solutions with pH ranging between 6.8
and 7.4, but easily hydrolysed at extreme

Form Five Student’s Book
72